Paper is typically manufactured from cellulose fibers which are extracted from a number of sources, principally wood and recycled paper. The various sources and processes for creating and separating the individual wood fibers results in a paper stock containing contaminants which must be removed before the wood fibers can be used to make paper. While many contaminants can be removed from the fiber stock by screening, other contaminants are of a size which makes their removal by filtration difficult. Historically, hydrocyclones or centrifugal cleaners of relatively small size, normally from 2-72 inches in diameter have been employed. It has been found that the centrifugal type cleaner is particularly effective at removing small area debris such as broken fibers, cubical and spherical particles, and seeds, as well as non-woody fine dirt such as bark, sand, grinderstone grit and metal particles.
The relatively small size of the centrifugal cleaners allows the employment of certain hydrodynamic and fluid dynamic forces provided by the combination of centrifugal forces and liquid shear planes produced within the hydrocyclone which allows the effective separation of small debris.
The advent of certain modern sources of pulp fibers such as tropical wood species and recycled paper which is contaminated with stickies, waxes, hot melt glues, polystyrenes, polyethylenes, and other low density materials including plastics and shives presents additional problems in the area of stock preparation. The ability of the hydrocyclone to separate both high density and low density contaminants gives them particular advantages in dealing with the problem of cleaning modern sources of paper fiber. Many modern fiber sources tend to be contaminated with both heavyweight and lightweight contaminants.
In one common type of forward cleaner, the flow of acceptable material must change direction at the bottom of the cleaner and travel back up to the top. Such a cleaner also has little control on changing the reject flow volume. To limit the amount of good fiber lost, it is necessary to restrict the volume of material rejected. This usually requires that the rejects orifice be small and in the center of the cleaner. Various systems using elutriation water have also been tried, but it is fed from the outside diameter of the rejects area. Rejects volume in these cases would be controlled by elutriation water pressure and rejects flow control valves which are expensive on small cleaners and need to be carefully monitored.
While existing hydrocyclones have been developed to remove both heavy and light contaminants, further improvements in this area are highly desirable. The fact that each hydrocyclone is a small device, and they are therefore used in banks of up to sixty or more cleaners, means that each hydrocyclone must be of extremely high reliability and require minimal maintenance or the entire hydrocyclone system will have poor reliability and high maintenance costs. One particular problem with hydrocyclones which can aggravate the reliability and maintenance problems is that separation effectiveness increases as the size or rate of the reject flow increases. However, increasing the reject flow increases the rejection of good fiber. The rejection of good fiber, in turn, requires additional stages for the recovery and separation of the rejected good fiber. Decreasing the size of the rejection flow to decrease the rejection of good fiber typically leads to two problems: Loss of separation effectiveness and clogging of the hydrocyclone with sand and grit. Furthermore, because the heavyweight rejects flow is typically small compared to the total throughput of the cleaner, prior art cleaners present the possibility of very slow heavyweight reject flows which are more likely to clog the cleaner.
What is needed is a stock cleaner of increased effectiveness, while retaining acceptable reliability and fiber utilization.